Mass spectrometers are an apparatus for ionizing molecules and atoms of a sample component included in a gaseous, liquid or solid sample, and separating the ions in every mass-to-charge ratio to detect them in order to identify the sample component or determine the component amount. It is widely used today for a variety of purposes such as the determination of biological samples and analysis of protein or peptide.
In the fields of biochemistry and medicine, which treat living organisms, there is a great demand for obtaining the distribution information of protein included in a cell in vivo without destroying the cell. In order to meet such a demand, a mass microscope which has both the function of a microscope and that of a mass spectrometer has been developed in many places. With a mass microscope, it is possible to obtain information about a substance's distribution or other data in a two-dimensional area on a sample set on a preparation or the like.
FIG. 6 is a schematic configuration diagram of a conventional and general mass spectrometer of this kind. To the sample 4 placed on the sample stage 3 which is movable in biaxial directions of the x-axis and y-axis, a laser beam 2 with a narrow diameter for ionization is irradiated from the laser irradiator 1 for only a short period of time. In response to the laser irradiation, the sample components included in the sample 4 are ionized, and the ions generated are introduced to the mass separator 5 to be separated in every mass-to-charge ratio. Then, they are detected by the ion detector 6. In this figure, it is assumed that the mass separator 5 is a time-of-flight (TOF) mass separator which separates ions in every mass-to-charge ratio in accordance with the flight time difference; however, a mass separator of another configuration such as a quadrupole mass filter may be used.
In this configuration, the area on the sample 4 that can be mass-analyzed by one laser irradiation is very small. Hence, in order to perform a mass analysis for the entire sample 4 or across a rather wide area on the sample 4, a laser irradiation and a mass analysis corresponding thereto are repeated while the sample stage 3 is two-dimensionally moved by a stage drive unit which is not shown. With this operation, each piece of mass spectrum information which corresponds to a small region on the sample 4 is obtained, and based on this information, a two-dimensional image which illustrates a substance distribution or other information is created. Such a two-dimensional image will be hereinafter called “a two-dimensional substance distribution image.”
With the aforementioned configuration, in the case where the two-dimensional area to be analyzed is large, the number of repeated analysis tasks will be enormous, and it takes a long time to obtain a two-dimensional substance distribution image. In addition, although the spatial resolution of a two-dimensional substance distribution image is determined by the laser irradiation area, the laser cannot be narrowed down to a diameter approximately a few dozen μm on the sample surface with current technology. Such a spatial resolution is not enough to observe a living cell or the like, and spatial resolution is required to be enhanced by one more orders of magnitude. However, it is difficult to achieve this only by modifying a lens optical system or other units for narrowing down the laser irradiation diameter. Even if the laser irradiation diameter can be narrowed down, another problem will arise: the amount of ion generation decreases since the area to be analyzed is small, which leads to the decrease of the analysis accuracy.
On the other hand, in terms of shortening the analysis time, a mass spectrometer having a configuration illustrated in FIG. 7 has also been proposed (see Non-Patent Document 1). That is, a large area on the sample 4 is irradiated with a planer laser light 12 by the laser irradiator 11 for a short period of time, and the sample components included in the sample 4 are ionized all together from the large area. Then, each ion is introduced to the time-of-flight mass separator 15 so that the ions retain the relative relationship of the ions' generation positions on the sample 4. After that, with the relative relationship being maintained, various kinds of ions generated from the same position are separated according to the mass-to-charge ratio and then detected by the two-dimensional detector 16. This configuration allows a mass analysis of the entire sample 4 or a relatively large area of the sample 4 with one laser irradiation.
However, with this configuration, the relative relationship of the ions' generation positions on the sample 4 is required to be maintained also on the detector plane of the two-dimensional detector 16. Although the scaling of the image can be performed according to necessity, in practice it is difficult to perform an ion transport which completely satisfies such a condition. In the case where the condition is not satisfied, the spatial resolution of a two-dimensional substance distribution image decreases, which makes the image blur. In the case where a sample including a sample component with a relatively large molecular weight is analyzed, there is a demand in some cases that the ions generated from the sample 4 are dissociated once or plural times to be fragmentized and then mass analyzed. For that purpose, an ion trap, a collision induced dissociation cell or other units are required to be placed along the ion pathway; however, this spoils the previously-described relative relationship of the ions' generation positions.
[Non-Patent Document 1] Yasuhide Naito, “Mass Microprobe Aimed at Biological Samples,” Journal of the Mass Spectrometry Society of Japan, volume 53, no. 3, 2005.